1. Technical Field of The Invention
The present invention relates to a steam cooled stationary blade for a gas turbine, and more particularly to a cooled stationary blade for a gas turbine for steam cooling both an inner shroud and the blade.
2. Description of The Related Art:
FIG. 5 shows a typical conventional air cooled type gas turbine stationary blade. In this drawing, numeral 40 denotes a stationary blade, numeral 41 denotes an outer shroud and numeral 42 denotes an inner shroud. Reference characters 43A, 43B, 43C, 43D and 43E denote respective air passages. Numeral 45 denotes a rear edge of the blade. Numeral 44 denotes air blowout holes at the rear edge. Reference numeral 46 denotes turbulators provided in an inner wall of each air passage 43A to 43E for enhancing heat transmission by distributing the air flow.
In this air cooled type stationary blade, the cooling air 47 is introduced from the outer shroud 41 to the air passage 43A and flows to a base portion (at the inner shroud side). The cooling air is introduced from the base portion into the next air passage 43B. The cooling air flows to an upper end (at the outer shroud side) and into the next air passage 43C. The cooling air flows in the same way through the air passages 43D and 43E, in that order, to thereby cool the blade. Then, in the air passage 43E, the cooling air is blownout from the air blowout holes 44 of the rear edge 45, and at the same time, the rest of the air flows out from the lower side of the inner shroud 42.
In the above air cooled type stationary blade, a serpentine cooling path is formed by the air passages 43A to 43E to cool the blade by means of the cooling air flowing through the path. However, there is no consideration of the cooling effect on the shrouds.
FIG. 4 shows an example of a cooled stationary blade in which the blade is cooled by steam and the shrouds are cooled by air. The steam cooling system used in this stationary blade has not yet been put into practical use. However, it is a technique which has been researched by the present applicant. In the drawing, reference numeral 30 denotes the stationary blade, from which the outer shroud at an upper portion thereof has been omitted, and in which a portion of the blade is shown. Numeral 31 denotes the inner shroud. Reference numerals 33A, 33B, 33C, 33D, 33E and 33F denote steam passages of the respective interiors of the stationary blade.
In the thus constructed stationary blade, the cooling steam 39 is introduced from a front edge portion of the outer shroud (not shown) to the steam passage 33A and from a base portion thereof (inner shroud side) into the steam passage 33B. The cooling steam flows from an upper portion of the steam passage 33B (at the outer shroud side) into the next steam passage 33C and flows through the steam passages 33D and 33E in a similar manner. The steam flows from the base portion side of the steam passage 33E into the steam passage 33F on the rear edge side to cool the interior of the blade. Thereafter, the steam is recovered from the steam recovery port of the outer shroud.
On the other hand, the inner shroud 31 is cooled by cooling air.
The cooling air 37, introduced from the lower portion of the inner shroud 31, is introduced into air cooling passages in the interior of the inner shroud 31 from one end thereof. The air flows from one side to the other within these air cooling passages to cool the entire inner shroud 31 and is discharged from the air blowout holes 38 on the other side to air cool the entire blade.
As described above, in the conventional gas turbine stationary blade shown in FIG. 5, the air cooling system is mainly used to cool the blade, but not to cool the inner shroud at all. Also, in the air cooling system shown in FIG. 4, in an example made by the present applicant, the cooling air is introduced into the air cooling passages within the inner shroud 31 and flows from one side to the other in the inner shroud to cool the surface of the shroud from the interior. The air flows out from the air blowout holes 38 on the other side. Furthermore, although not shown in this case, a recess is formed in the inner surface of the inner shroud 31. An impingement plate is provided in parallel with the inner surface of the inner shroud. Another (method) also being developed by the present applicant is one in which the cooling air 37 fed from the lower portion impinges on the impingement plate and is blownout from a number of holes so that the interior of the shroud is uniformly cooled by the air.
However, in the air cooling system shown in FIG. 5 described above, a large amount of air is consumed for cooling and the air that has been used for cooling is discharged to the combustion gas passage. Consequently, the system suffers from a problem in that a relatively large amount of power is consumed by a compressor or a cooler. Also, since the air that has been used for cooling is discharged into the combustion gas passage, the cooling air is mixed with the combustion gas which lowers the gas temperature resulting in a reduction of turbine efficiency.
On the other hand, in the steam cooling system for the blade shown in FIG. 4, since the blade is cooled by using steam and the steam which has been used for cooling is recovered and returned to the steam feed source, it is possible to utilize the steam effectively. However, only the blade is cooled by the steam, and the air cooling system is used for the inner shroud. The air that has been used for cooling the inner shroud is discharged into the main stream of the combustion gas flowing through the gas turbine. Accordingly, compared with the system of cooling the blade with air as shown in FIG. 5, it is possible to conserve and reduce the amount of cooling air. However, in any case, the turbine efficiency is lowered because the cooling air is needed and the temperature of the combustion gas is lowered by the mixture of the air into the combustion gas.